Sylvester Stallone plays Stakar Ogord who becomes the hero Starhawk.

Here's how Stakar's look as Starhawk has changed since his introduction in the "Defenders" comic.
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Stakar was raised by the Reavers, a group of cyborgs. In "Vol. 2" he's a respected leader of the Ravagers, the gang of space pirates from the first film which includes Yondu. In the comics, the character is also known as Starhawk, a hero who has worked alongside the Fantastic Four and Thor and who has a long lifespan.

We didn't see Stallone with any of the comic character's flying abilities so it looks like he may not be Starhawk yet.

Aleta was sexualized in the comics when first introduced. The image on the left is from a "pin-up" page of the character. Her costume these days is better suited for fighting.
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Aleta's relationship to Stakar is an interesting one. The two grew up as adopted brother and sister before falling in love and getting married. However, the two wound up merged together and trapped in the same body after coming across a Hawk God. Together, they became the superhero Starhawk. Only one of them is able to have control of their body at once, and, for the most part, Stakar is who we see as Starhawk.

They were allowed to be separated at one point to have children; however, they were killed by Aleta's father. From that point on, Aleta always held Stakar responsible. When the two were finally separated, she moved on with another Guardians member, Vance.

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Ving Rhames plays tough guy Charlie-27.

Charlie-27 in his first comic appearance (left) and in the 2014 "Guardians" comics.
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Charlie-27 is a space pilot for Jupiter who has teamed up with a lot of different heroes and groups like Thor and Captain America, time-traveling through comics. He also becomes involved in the "Infinity War," which will be the next "Avengers" movie.

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Krugarr's a member of the Lemian race, who are red-skinned aliens.

Here's Krugarr with the Ancient One, a character from Doctor Strange. Krugarr is wearing the cape that Doctor Strange currently wears in the Marvel movies.
Marvel

Krugarr is a wormlike creature who is the Sorcerer Supreme in an alternate reality (Sorry Doctor Strange). In the end-credits scene we see him using magic similar to Strange.

Krugarr first appeared in "Guardians of the Galaxy Annual" #1 in 1991. We haven't heard yet if anyone voiced Krugarr.

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Miley Cyrus voiced Mainframe, an android created by Tony Stark.

We only saw Mainframe without a body.
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There are multiple versions of Mainframe in the comics, but one of them is a future version of Vision, Tony Stark's AI that became sentient, on an alternate Earth. When Stark wanted to retire he made an android using Iron Man armor and that became Mainframe. Even if destroyed, a beacon inside of Mainframe will upload his consciousness into a replacement suit back at Stark's location. Pretty cool.

"I was watching 'The Voice' and I [thought] she's so likable and her voice is awesome, she's got the best voice," Gunn told reporter Sarah Ruhlman. "And then I asked Kevin [Feige], what do you think about casting Miley Cyrus as the voice of Mainframe? He was like well if you think we can get her … and I got her to do it."

Michael Rosenblaum voices the crystallized Martinex T'Naga.

You may recognize Rosenbaum for his role on "Smallville." The above image is Martinex from 1991's "Guardians of the Galaxy Annual" #2.
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Martinex is from Pluto so his skin is covered in a crystal layer which allows him to survive in the planet's cold temperatures. He doesn't need air to survive and he can see in the dark. An alien race called the Badoon destroyed his people while he was away from the planet briefly.

We see Martinex (Marty for short) hanging with Stakar in "Guardians of the Galaxy Vol. 2."

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Bonus: Yondu (Michael Rooker) was the leader of the original Guardians.

Yondu had some colorful outfits when the Guardians debuted in 1969 and in the early '90s, but he has always had his fin.
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After Yondu's race was destroyed he teamed up with Charlie-27 and Martinex, who also lost their people to the Badoon, to form the Guardians of the Galaxy and take down the alien race.

And, just like in the sequel, Yondu and Rocket Raccoon really have teamed up in the past. While with Rocket in "Vol. 2," Yondu briefly says that he once was part of a crew like Peter Quill's, a nod to his Guardians team.